PROJECT SUMMARY/ABSTRACT The postnatal kidney has an extremely limited ability to transport organic anions, whereas the adult kidney (proximal tubule) has a very high capacity organic anion (PAH) transport system. These include a wide variety of small organic molecules including metabolites (eg. carnitine), dietary compounds (eg. flavonoids), signaling molecules (eg. short chain fatty acids, odorants), antioxidants (eg. urate), drugs (eg. analgesics) and toxins (eg. mercurials). The renal organic anion transport system includes the following transporters, among others: OAT1 (first discovered by the PI's lab as NKT), OAT3, MRP2 and MRP4. The OATs appear to be the rate- limiting step in renal elimination. The question we are asking is: How does this transformation occur such that there is almost no organic anion transport in the neonate to a very high capacity organic anion transport system? Our metabolomics studies of the OAT1 and OAT3 indicate that the OATs are the main routes of renal handling of a wide range of gut microbiome-derived metabolites (which are also organic anions). Time series analysis (postnatal to adult) indicates that these metabolites are present early in postnatal plasma/urine and that their elimination is sensitive to the pan-OAT inhibitor probenecid. Since the organic anion transporter system is known to be inducible by other OAT substrates during a post-natal developmental window, we propose that, under normal conditions, it is these gut microbiome products that induce the expression of OATs (and possibly MRPs) and thus functional capacity during the postnatal developmental window. This ensures a high capacity organic anion transport system in the adult proximal tubule which is able to eliminate the aforementioned small organic molecules as well as uremic solutes. We propose to answer the following questions: SA1. a) What are the gut microbiome-derived endogenous plasma metabolites at each stage of postnatal development? b) What is the relationship of the changing gut flora (postnatal to adult) to the handling of gut microbiome-derived products in the maturing kidney? (time series of 16S gut flora sequencing in the context of metabolomics time series data)? SA2. a) Does absence of the gut flora in the newborn during the substrate-inducibility window diminish (or otherwise alter) adult handling of classic substrates of the renal organic anion transport system (PAH, estrone sulfate)? If we are able to quantitatively prove our hypothesis-- that early exposure to gut microbiome-derived products is essential to proper organic anion transport in the adults, this would be a major advance for the field and also set the stage for new approaches to enhancing tubular function (e.g., premature infant, early stages of kidney disease).